A number of existing electronic ballasts for gas discharge lamps include an inverter and a series resonant output circuit that provide a high voltage for igniting the lamps and a high frequency current for efficiently powering the lamps.
A series resonant output circuit draws a certain amount of input current from the inverter. Apart from that portion of the input current that is needed for powering the lamps, a considerable portion of the input current is reactive in nature and is commonly referred to as "circulating current." In "driven" type inverters that do not utilize resonant current feedback for commutating the inverter switches, the circulating current serves no useful purpose after the lamps are ignited and has the undesirable effect of increasing the power dissipation in the resonant circuit and in certain inverter components such as the inverter switches.
The power losses due to circulating current degrade ballast energy efficiency and reliability, and are a significant impediment to efforts to reduce the physical size, weight, and cost of the ballast itself. These problems are particularly acute for higher power ballast circuits, such as those for driving three or four lamps, for which miniaturization efforts are highly contingent upon reduced power losses in the inverter and output circuit.
In addition to the problem of circulating current, many existing electronic ballasts that employ a driven type inverter have the disadvantage of requiring special protection circuitry to prevent self-destruction of the inverter and resonant output circuit in the event of lamp failure or lamp fault conditions. Such protection circuitry is often very extensive and quite complex, and adds considerably to the resulting physical size and cost of the ballast.
It is therefore apparent that a need exists for an electronic ballast that substantially reduces or eliminates circulating current and that lessens or obviates the need for extensive protection circuitry. Such a ballast would be significantly more energy efficient than existing ballasts, would be amenable to reductions in size, weight, and cost, and would therefore represent a considerable advance over the prior art.